U.S. patent application number 15/371497 was filed with the patent office on 2017-03-23 for mechanical shock resistant mems accelerometer arrangement, associated method, apparatus and system.
The applicant listed for this patent is Merlin Technology, Inc.. Invention is credited to Albert W. Chau, John E. Mercer, Scott Phillips.
Application Number | 20170082654 15/371497 |
Document ID | / |
Family ID | 55016840 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170082654 |
Kind Code |
A1 |
Chau; Albert W. ; et
al. |
March 23, 2017 |
MECHANICAL SHOCK RESISTANT MEMS ACCELEROMETER ARRANGEMENT,
ASSOCIATED METHOD, APPARATUS AND SYSTEM
Abstract
An accelerometer arrangement and method are described for
determining accelerations of an inground tool. First and second
triaxial accelerometers are supported such that a normal sensing
axis of the first triaxial accelerometer is at least generally
orthogonal to the normal sensing axis of the second triaxial
accelerometer for determining the accelerations along the three
orthogonal axes based on a combination of sensing axis outputs from
one or both of the triaxial accelerometers. A weaker sensing axis
of one triaxial accelerometer can be supported at least
approximately normal to a weaker sensing axis of another triaxial
accelerometer such that the weaker axes are not used. The triaxial
accelerometers can be supported such that one axis of one
accelerometer can be redundant with respect to another axis of
another accelerometer. One triaxial accelerometer can be mounted on
a tilted plane with respect to another triaxial accelerometer.
Inventors: |
Chau; Albert W.;
(Woodinville, WA) ; Mercer; John E.; (Gig Harbor,
WA) ; Phillips; Scott; (Kent, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Merlin Technology, Inc. |
Kent |
WA |
US |
|
|
Family ID: |
55016840 |
Appl. No.: |
15/371497 |
Filed: |
December 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14789071 |
Jul 1, 2015 |
9551730 |
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15371497 |
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62021618 |
Jul 7, 2014 |
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62019887 |
Jul 2, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01P 15/18 20130101;
G01P 15/0888 20130101; G01P 15/00 20130101; G01P 3/00 20130101;
B81B 2201/0235 20130101 |
International
Class: |
G01P 15/18 20060101
G01P015/18 |
Claims
1. An accelerometer arrangement configured for determining
accelerations of an inground tool along three orthogonal axes
during an inground operation that exposes the accelerometer
arrangement to a mechanical shock and vibration environment, said
accelerometer arrangement comprising: a first MEMS triaxial
accelerometer and a second MEMS triaxial accelerometer, each of
which includes a set of three orthogonally arranged accelerometer
sensing axes including a pair of in-plane sensing axes and a normal
sensing axis such that each normal sensing axis is subject to a
higher rate of failure responsive to mechanical shock and vibration
than the in-plane sensing axes and the first and second MEMS
triaxial accelerometers are supported with the normal sensing axis
of the first MEMS triaxial accelerometer at least generally
orthogonal to the normal sensing axis of the second MEMS triaxial
accelerometer; and a processor configured to determine the
accelerations along said three orthogonal axes of the inground tool
based on a combination of sensing axis outputs from the first and
second triaxial accelerometers without using normal sensing axis
outputs of each of the first and second triaxial
accelerometers.
2. The accelerometer arrangement of claim 1 supported within a
transmitter that is carried by the inground tool.
3. The accelerometer arrangement of claim 2 wherein the transmitter
defines an elongation axis and one sensing axis of the first
triaxial accelerometer and another sensing axis of the second
triaxial accelerometer are at least generally parallel to the
elongation axis.
4. The accelerometer arrangement of claim 3 wherein the normal
sensing axis of one of the first and second MEMS triaxial
accelerometers is at least generally parallel to the elongation
axis.
5. The accelerometer arrangement of claim 2 wherein at least one
in-plane sensing axis of the first and second triaxial MEMS
accelerometers is arranged for sensing a pitch orientation of the
transmitter.
6. The accelerometer arrangement of claim 2 wherein the pair of
in-plane sensing axes of one of the first and second tri-axial
accelerometers is supported for detecting a roll orientation of the
transmitter.
7. The accelerometer arrangement of claim 2 wherein said
transmitter includes a first printed circuit board that supports
the first triaxial accelerometer and a second printed circuit board
that supports the second triaxial accelerometer.
8. The accelerometer arrangement of claim 7 wherein the second
printed circuit board is supported by the first printed circuit
board at least generally orthogonal thereto.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application of copending
U.S. patent application Ser. No. 14/789,071, filed on Jul. 1, 2015,
which claims priority from U.S. Provisional Patent Application Ser.
No. 62/019,887, filed on Jul. 2, 2014, and U.S. Provisional Patent
Application Ser. No. 62/021,618, filed on Jul. 7, 2014, the
disclosures of which are incorporated herein by reference.
BACKGROUND
[0002] The present application is generally related to MEMS
accelerometers and, more particularly, to a MEMS accelerometer
package having enhanced resistance to mechanical shock as well as
an associated method, apparatus and system.
[0003] Accelerometers have become ubiquitous in modern electronic
devices. In this regard, the measurement of acceleration along
three orthogonal axes can provide for essentially complete
characterization of device orientation. An accelerometer that is
configured for measuring accelerations along three orthogonal axes
in a single, convenient package is often referred to as a triaxial
accelerometer.
[0004] Many modern electronic systems employ accelerometers for a
wide range of purposes. As a general example, the operational state
or physical orientation of an associated device can be
characterized. As another more device specific example, an
accelerometer can be used to detect that a hard disk drive is in a
state of free fall such that the read/write heads of the drive can
be parked in anticipation of mechanical shock upon impending
impact. As still another device specific example, modern cellular
smart phones typically include an accelerometer to determine the
orientation of the phone for display orientation management and for
use by a wide range of applications that can be installed on the
smart phone. It is noted that the need for accelerometers suited
for consumer-grade electronics such as cellular smart phones has
driven the development of MEMS triaxial accelerometers that are
typically low-cost.
[0005] As still another device specific example, an accelerometer
can be used as part of a transmitter that is carried by an inground
tool in a horizontal directional drilling system for monitoring the
orientation and movement of the inground tool. Such monitoring can
facilitate steering as well as monitoring the position of the
inground tool. As will be described in detail below, Applicants
recognize that the use of consumer-grade low-cost accelerometers in
a device that subjects the accelerometer to a mechanical shock and
vibration environment can lead to failures of these devices. While
the overall failure rate has historically not been high, any
premature failure of a transmitter can result in significant
problems, including idling a crew and equipment while a new
transmitter is obtained, missing deadlines, as well as the cost
involved with purchasing a new transmitter. To date, the industry
has continued to use these accelerometers for lack of an
identified, practical alternative.
[0006] The present application brings to light a new approach which
provides for the use of low-cost consumer-grade accelerometers in a
high mechanical shock and vibration environment in a way which
enhances reliability.
SUMMARY
[0007] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods
which are meant to be exemplary and illustrative, not limiting in
scope. In various embodiments, one or more of the above-described
problems have been reduced or eliminated, while other embodiments
are directed to other improvements.
[0008] In general, an accelerometer arrangement and method are
described for determining accelerations of an inground tool along
three orthogonal axes during an inground operation that exposes the
accelerometer arrangement to a mechanical shock and vibration
environment. In one aspect of the disclosure, the accelerometer
arrangement includes a first MEMS triaxial accelerometer and a
second MEMS triaxial accelerometer, each of which includes a set of
three orthogonally arranged accelerometer sensing axes including a
pair of in-plane sensing axes and a normal sensing axis such that
the normal sensing axis is subject to a higher rate of failure
responsive to mechanical shock and vibration than the in-plane
sensing axes. A support structure supports the first and second
triaxial accelerometers such that the normal sensing axis of the
first triaxial accelerometer is at least generally orthogonal to
the normal sensing axis of the second triaxial accelerometer. A
processor determines the accelerations along the three orthogonal
axes based on a combination of sensing axis outputs from one or
both of the first and second triaxial accelerometers.
[0009] In another aspect of the disclosure, the accelerometer
arrangement includes a first MEMS triaxial accelerometer and a
second MEMS triaxial accelerometer, each of which includes a weaker
sensing axis that is more susceptible to mechanical shock and
vibration than the other two sensing axes. A support structure
supports the first and second triaxial accelerometers such that the
weaker sensing axis of the first triaxial accelerometer is at least
approximately normal to the weaker sensing axis of the second
triaxial accelerometer. A processor determines the accelerations
along the three orthogonal axes based on a combination of sensing
axis outputs from the first and second triaxial accelerometers
without using the weaker sensing axis of each of the first and
second triaxial accelerometers.
[0010] In still another aspect of the disclosure, the accelerometer
arrangement includes a first accelerometer package and a second
accelerometer package, each of which includes one or more sensing
axes such that the first and second accelerometer packages
collectively provide a total of at least four sensing axes for
sensing along the three orthogonal axes. A support structure
supports the first and second accelerometers such that at least one
sensing axis of the first accelerometer package is redundant with
respect to at least one sensing axis of the second accelerometer
package. A processor is configured to select a combination of three
sensing axes from the total number of sensing axes for determining
the accelerations along the three orthogonal axes.
[0011] In yet another embodiment of the present disclosure, the
accelerometer arrangement includes a first MEMS triaxial
accelerometer and a second MEMS triaxial accelerometer. A support
structure supports the first and second triaxial accelerometers
such that the first triaxial accelerometer is supported on a first
plane that forms an angle of at least approximately 45 degrees with
respect to a second plane that supports the second triaxial
accelerometer. A processor determines the accelerations along the
three orthogonal axes based on a combination of sensing axis
outputs from the first and second triaxial accelerometers.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0012] Example embodiments are illustrated in referenced figures of
the drawings. It is intended that the embodiments and figures
disclosed herein are to be illustrative rather than limiting.
[0013] FIG. 1 is a diagrammatic view, in elevation, of a
transmitter, according to the present disclosure, which utilizes
dual triaxial MEMS accelerometers.
[0014] FIG. 2 is a diagrammatic perspective view of an embodiment
of the accelerometer arrangement of the transmitter of FIG. 1.
[0015] FIG. 3 is a schematic diagram illustrating an embodiment of
the transmitter of FIG. 1.
[0016] FIG. 4 is a flow diagram that illustrates an embodiment for
the operation of the transmitter of FIG. 1.
[0017] FIGS. 5 and 6 are diagrammatic views, in perspective, of an
embodiment of the accelerometer arrangement of FIG. 1.
[0018] FIG. 7 is a flow diagram illustrating an embodiment of a
method for operating a transmitter in accordance with the present
application based on a priority table of sensing axis combinations
derived from two or more accelerometers.
[0019] FIG. 8 is a diagrammatic illustration, in a perspective
view, of another embodiment for supporting dual accelerometers in
accordance with the present disclosure, wherein the accelerometers
are supported on planes that are non-normal with respect to one
another.
DETAILED DESCRIPTION
[0020] The following description is presented to enable one of
ordinary skill in the art to make and use the invention and is
provided in the context of a patent application and its
requirements. Various modifications to the described embodiments
will be readily apparent to those skilled in the art and the
generic principles taught herein may be applied to other
embodiments. Thus, the present invention is not intended to be
limited to the embodiment shown, but is to be accorded the widest
scope consistent with the principles and features described herein
including modifications and equivalents, as defined within the
scope of the appended claims. It is noted that the drawings are not
to scale and are diagrammatic in nature in a way that is thought to
best illustrate features of interest. Descriptive terminology may
be used with respect to these descriptions, however, this
terminology has been adopted with the intent of facilitating the
reader's understanding and is not intended as being limiting.
Further, the figures are not to scale for purposes of illustrative
clarity.
[0021] Turning now to the figures wherein like components are
indicated by like reference numbers throughout the various figures,
attention is immediately directed to FIG. 1 which is an elevational
view that diagrammatically illustrates an embodiment of a
transmitter, generally indicated by the reference number 10, for
use in a horizontal directional drilling system. Transmitter 10 is
supported by an inground tool 14 such as, for example, a boring
tool for performing a drilling operation or a tension monitoring
arrangement used with a back-reaming tool for performing a
pull-back operation. As will become apparent, the present
disclosure is relevant with respect to any application which
subjects certain components to a mechanical shock and vibration
environment.
[0022] Transmitter 10 includes a processor 20 in communication with
a transmitter section 22 that is coupled to an antenna 24 for
emitting a signal 28 such as, for example, a locating signal which
can be a dipole signal. The transmitter can include an elongation
axis 30 that can at least generally align with an elongation axis
of inground tool 14 when the transmitter is installed therein. It
is noted that inter-component cabling is not shown in FIG. 1 for
purposes of illustrative clarity, but is understood to be present.
In another embodiment, transmitter section 10 can comprise a
transceiver for also receiving an incoming electromagnetic signal.
In still another embodiment, above ground communication can be
implemented by using a drill string (not shown) that is attached to
inground tool 14 as an electrical conductor as described, for
example, in U.S. Published Application no. 2013/0176139 which is
hereby incorporated by reference in its entirety. It should be
appreciated that the teachings herein remain applicable
irrespective of the particular communication pathway or pathways
that are implemented. Any appropriate combination of sensors can be
provided as part of the transmitter such as, for example, a
pressure sensor 32, a temperature sensor 36 and an accelerometer
arrangement 40. Data collected from these various sensors by
processor 20 can be transmitted, for example, by modulating signal
28.
[0023] Referring to FIG. 2 in conjunction with FIG. 1, in one
embodiment, accelerometer arrangement 40 comprises a MEMS
accelerometer arrangement that is supported within transmitter 10.
The MEMS accelerometer arrangement includes a first triaxial MEMS
accelerometer 44a and a second triaxial MEMS accelerometer 44b. It
is noted that these accelerometers may be referred to collectively
by the reference number 44. FIG. 2 is an enlarged diagrammatic
view, in perspective, of MEMS accelerometer package 40. First
triaxial MEMS accelerometer 44a includes orthogonal sensing axes
x.sub.1, y.sub.1 and z.sub.1, while second triaxial MEMS
accelerometer 44b includes orthogonal sensing axes x.sub.2, y.sub.2
and z.sub.2. It is noted that axes x.sub.2 and z.sub.1 can be at
least approximately aligned or parallel with elongation axis 30,
although this is not required. The triaxial accelerometers can be
supported and electrically connected in any suitable manner. In the
present example, a first printed circuit board 50 supports and
electrically connects triaxial MEMS accelerometer 44a while a
second printed circuit board 52 supports and electrically connects
second triaxial MEMS accelerometer 44b. Generally, the same part
number can be used for both triaxial MEMS accelerometers, although
this is not required. The triaxial MEMS accelerometers can be
inexpensive, consumer-grade MEMS accelerometers such as, for
example, those that are found in cellular telephones. One
non-limiting example of such a consumer-grade triaxial
accelerometer is the MMA 8451Q manufactured by Freescale
Semiconductor. In the present embodiment, accelerometers 44 are
configured with an I2C interface such that processor 20 accesses
readings along each axis by reading from specific memory locations
within each triaxial accelerometer, although any suitable form of
interface, including analog, can be utilized.
[0024] Triaxial MEMS accelerometers have become the most widely
available, and in many cases least expensive, accelerometers
available in the market. These accelerometers have become pervasive
in applications ranging from cellular telephones to toys. However,
underground drilling can subject accelerometers to higher and/or
more prolonged levels of mechanical shock that may in some
circumstances exceed the thresholds for which these components are
typically designed. In particular, Applicants recognize that the
z-axis, which can be referred to as a normal axis or a weaker axis,
of triaxial MEMS accelerometers tend to fail more often than other
axes, which can be referred to as the in-plane axes, when exposed
to extreme shock conditions, thereby limiting the overall
performance of the component. Manufacturers of locating systems use
these components in spite of this limitation in the absence of a
more suitable alternative. The teachings that are brought to light
herein compensate for this limitation, providing the potential for
a significantly higher level of reliability when these
accelerometers are used in underground drilling applications or
other conditions of extreme mechanical shock.
[0025] As best seen in the perspective view of FIG. 2, MEMS
accelerometer 44a is mounted orthogonal to MEMS accelerometer 44b
such that the z.sub.1 axis of accelerometer 44a is at least
approximately orthogonal to the z.sub.2 axis of accelerometer 44b
and the x.sub.1, y.sub.1 plane is at least approximately orthogonal
to the x.sub.2, y.sub.2 plane. It is noted that the x and y sensing
axes of a given accelerometer may be referred to as the in-plane
axes since these axes define a plane that is at least generally
parallel to the surface on which the triaxial accelerometer package
is supported, while the z axis may be referred to as a normal axis.
In an embodiment, accelerometer readings for three orthogonally
disposed axes can be obtained based on sensing along axis x.sub.1
of accelerometer 44a as well as along axes x.sub.2 and y.sub.2 of
accelerometer 44b such that the x.sub.1 accelerometer readings are
used in place of z.sub.2 readings from accelerometer 44b. In
another embodiment, accelerometer readings can be obtained based on
sensing along axes x.sub.1 and y.sub.1 of accelerometer 44a and
along axis x.sub.2 of accelerometer 44b such that the x.sub.2
accelerometer readings are used in place of z.sub.1 readings from
accelerometer 44a. In either of these embodiments, no sensor
readings are needed from the z axes of the dual accelerometers.
Stated in another way, two triaxial MEMS accelerometer packages can
be mounted normal or orthogonal to one another, thereby allowing
the normal accelerometer in one package to be replaced by an
in-plane accelerometer in the other package. This solution provides
for more than the simple redundancy of using multiple
accelerometers in the standard manner (i.e. using all three axes),
since this would still expose the device to potential failure of
the z-axis. By contrast, Applicant's configuration is specifically
designed to eliminate the recognized weakest link in these
components--namely, the z-axis.
[0026] Referring to FIG. 3, a schematic diagram of an embodiment of
transmitter 10 is illustrated. Descriptions of like components will
not be repeated for purposes of brevity. In the present embodiment,
an I2C interface 100 connects triaxial MEMS accelerometers 44a and
44b to CPU 20. Pressure sensor 32 and temperature sensor 36 can be
interfaced to processor 20 using an analog to digital converter
104, if either sensor is an analog sensor.
[0027] FIG. 4 is a flow diagram that illustrates one embodiment of
a method, generally indicated by the reference number 200, for the
operation of transmitter 10 in accordance with the present
disclosure. The method begins at start 204 and proceeds to 208
which reads axis x.sub.1 of accelerometer 44a. At 210, axis x.sub.2
of accelerometer 44b is read followed by reading axis y.sub.2 of
accelerometer 44b at 212. The various accelerometer axes can be
read individually, in any suitable order or combination, based on
the parameter or parameters that are being determined. Such
parameters include, by way of non-limiting example, pitch and roll
of inground tool 14. At 216, the accelerometer readings are used to
perform determinations such as, for example, pitch and/or roll
orientation parameters.
[0028] FIGS. 5 and 6 are diagrammatic views, in perspective of an
embodiment of accelerometer arrangement 40, shown here for further
illustrative purposes.
[0029] It should be appreciated that in addition to providing a
robust accelerometer arrangement using low-cost triaxial MEMS
accelerometers, still further benefits are provided by the
foregoing embodiments. For example, redundancy can be provided with
respect to accelerometer readings. Referring to FIG. 2, in an
embodiment that uses axes x.sub.1, x.sub.2 and y.sub.2, axis
y.sub.1 of accelerometer 44a is unused and can be used in place of
axis y.sub.2 of accelerometer 44b, for example, if a failure of
y.sub.2 is experienced.
[0030] The teachings that have been brought to light above can
readily be applied to other embodiments that are considered as
being within the scope of the present application so long as a
given embodiment practices avoiding the use of a weaker axis. For
example, in one embodiment, a dual accelerometer arrangement can
comprise a pair of dual axis accelerometers such that the normal,
z, or weaker axis is not present in either accelerometer and the
dual accelerometers are supported at least approximately normal or
orthogonal to one another such that an in-plane axis of one of the
dual axis accelerometers serves as a z or normal axis. In another
embodiment, a triaxial accelerometer can be paired with a dual axis
accelerometer such that an in-plane axis of the dual axis
accelerometer is supported to serve in place of the weaker, z, or
normal axis of the triaxial accelerometer. In still another
embodiment, a triaxial accelerometer can be paired with a single
axis accelerometer such that the single axis accelerometer is
supported to serve in place of the weaker, z, or normal axis of the
triaxial accelerometer. In yet another embodiment, a dual axis
accelerometer having a pair of in-plane accelerometers can be
paired with a single axis accelerometer such that the single axis
accelerometer is arranged at least approximately normal or
orthogonal to the in-plane accelerometers of the dual axis
accelerometer.
[0031] While the foregoing teachings provide for a much hardier
accelerometer package overall, additional robustness can be
obtained from the two tri-axial packages, as will be described
immediately hereinafter.
[0032] It should be appreciated that two tri-axial packages can
provide eight combinations of accelerometers that can serve as a
single tri-axial accelerometer with two accelerometers being
available for each Cartesian direction. Table 1 lists the
combinations in light of the accelerometer axes shown in FIG. 2. It
is noted that a set of master coordinate axes X,Y,Z are shown in
FIG. 2 such that the final column of Table 1 indicates the sign
associated with the respective master coordinate axis for each
sensing axis of a given combination.
TABLE-US-00001 TABLE 1 AVAILABLE COMBINATIONS OF AXES FOR DUAL
TRIAXIAL ACCELEROMETERS Combination Axes of Master no. Combination
Coordinate Axes Pitch O Roll .beta. 1* x.sub.2, y.sub.2, x.sub.1 X,
Y, Z sin.sup.-1 x.sub.2 tan.sup.-1 (x.sub.1/y.sub.2) 2 x.sub.2,
y.sub.1, z.sub.2 X, -Y, Z sin.sup.-1 x.sub.2 tan.sup.-1
(z.sub.2/y.sub.1) 3 z.sub.1, y.sub.2, z.sub.2 X, Y, Z sin.sup.-1
z.sub.1 tan.sup.-1 (z.sub.2/y.sub.2) 4* x.sub.2, y.sub.1, x.sub.1
X, -Y, Z sin.sup.-1 x.sub.2 tan.sup.-1 (x.sub.1/y.sub.1) 5 z.sub.1,
y.sub.2, x.sub.1 X, Y, Z sin.sup.-1 z.sub.1 tan.sup.-1
(x.sub.1/y.sub.2) 6 z.sub.1, y.sub.1, z.sub.2 X, -Y, Z sin.sup.-1
z.sub.1 tan.sup.-1 (z.sub.2/y.sub.1) 7 z.sub.1, y.sub.1, x.sub.1 X,
-Y, Z sin.sup.-1 z.sub.1 tan.sup.-1 (x.sub.1/y.sub.1) 8 x.sub.2,
y.sub.2, z.sub.2 X, Y, Z sin.sup.-1 x.sub.2 tan.sup.-1
(z.sub.2/y.sub.2) *= enhanced reliability
[0033] As denoted by asterisks in Table 1, there are two
combinations, 1 and 4, that provide the hardiest arrangement by
eliminating all use of the normal accelerometer in both
accelerometer packages.
[0034] For combinations 1, 2, 4 and 8, pitch, which is designated
as O, is given as:
O=sin.sup.-1 x.sub.2 EQN (1)
[0035] While roll, designated as .beta., for combinations 1 and 5
is given, by way of example, as:
.beta. = tan - 1 [ x 1 y 2 ] EQN ( 2 ) ##EQU00001##
[0036] And roll for combinations 2 and 6 is given, by way of
example, as:
.beta. = tan - 1 [ z 2 y 1 ] EQN ( 3 ) ##EQU00002##
[0037] Applicants recognize that the proper functionality of each
combination in Table 1 can be verified by summing the squares of
the three accelerations for each combination. The sum should equal
gravitational acceleration, g, squared. Representing the three
accelerometer readings for each combination generically using the
variables a, b and c:
g.sup.2=a.sup.2+b.sup.2+c.sup.2 EQN (4)
[0038] In actual practice, a range limit can be placed on the sum
of the squares of Equation 4 to account for accuracy of the
accelerometers and other measurement errors such that the
accelerometers associated with a particular sum of the Equation 4
can be deemed as operating correctly so long as the sum falls
between g.sub.min.sup.2 and g.sub.max.sup.2. Suitable values for
g.sub.min.sup.2 and g.sub.max.sup.2, by way of non-limiting
example, are at least approximately 0.958g.sup.2 and 1.05 g.sup.2,
respectively, or a change of +/-5 percent from 1 g.
[0039] In an embodiment, an ordered list of accelerometer
combinations, having the most reliable combinations at the top of
the list as a preference, can be utilized to determine which
accelerometer combination to use. Table 2 represents one embodiment
of such an ordered list wherein the combination numbers from Table
1 are set forth.
TABLE-US-00002 TABLE 2 PRIORITY TABLE FOR DUAL TRIAXIAL
ACCELEROMETERS Priority Combination Axes of Order no. Combination 1
1* x.sub.2, y.sub.2, x.sub.1 2 4* x.sub.2, y.sub.1, x.sub.1 3 2
x.sub.2, y.sub.1, z.sub.2 4 3 z.sub.1, y.sub.2, z.sub.2 5 5
z.sub.1, y.sub.2, x.sub.1 6 6 z.sub.1, y.sub.1, z.sub.2 7 7
z.sub.1, y.sub.1, x.sub.1 8 8 x.sub.2, y.sub.2, z.sub.2 *= enhanced
reliability
[0040] It should be appreciated that the use of a priority table
such as Table 2 does not require the use of one or more
accelerometers having a weaker axis. Priority assignments can be
made in accordance with any sort of concern that relates to
reliability. By way of non-limiting example, such concerns can
derive from the reliability of physical mounting, supporting
electrical connections, environmental exposure, and history. In
some embodiments, the priority table can be used even when the
accelerometer axis combinations are thought to all exhibit at least
generally the same reliability in order to provide an overall level
of reliability that is submitted to be heretofore unseen.
[0041] Referring to FIG. 7, an embodiment of a method is
illustrated for operating transmitter 10 based on a priority table
such as Table 2 is generally indicated by the reference number 200.
Method 200 begins at start 204, for example, when the transmitter
and accelerometers are first turned on and proceeds to 208. This
latter step sets each of a priority counter and a loop counter to a
value of 1. The purpose of the loop counter will be brought to
light at an appropriate point hereinafter. At 210, a sum of the
squares of the accelerometers for the first or highest order
combination of accelerometers in Table 2 is produced. At 214, the
sum of the squares value is tested against gmin.sup.2 and
gmax.sup.2, if the value is within range, operation is routed to
218 such that the current combination of accelerometers is used for
normal operation. During normal operation, the selected
accelerometer combination can be periodically monitored and/or
tested at 220 for failure, for example, based on Equation 4 and/or
any other suitable factors. If no failure is detected, normal
operation resumes at 224. If an accelerometer failure is detected
at 220, operation returns to 208 such that the procedure begins
anew.
[0042] Returning to the discussion of 214, if the sum of the
squares is out of range, operation proceeds to 230 which increments
the value of the priority order counter by 1. At 234, the value of
the priority order counter is tested against the total number of
available accelerometer combinations in Table 2. If the current
value of the priority order counter does not exceed the total
number of available combinations, operation returns to 210 and
proceeds therefrom. Otherwise, operation proceeds to 238 which
tests the current value of the loop counter against a loop count
limit. The purpose of the loop counter relates to the potential for
MEMS accelerometers to become temporarily stuck due to static
charge forces. Accordingly, it is not necessary to stop the
accelerometer selection procedure based on reaching the bottom of
the priority table list. Instead, the priority table list can be
looped through repeatedly some number of times before the
accelerometer package is declared unusable or the selection process
can continue indefinitely in hopes that the accelerometers become
functional. As part of the loop architecture, it should be
appreciated that every available combination of sensing axes can be
tested or re-tested including a combination that invoked the test
procedure in the first instance, for example, based on detection by
step 220. In this way, a previously failed combination that
subsequently becomes functional can be placed back into service. It
is noted that the test of step 238 and a loop architecture is not a
requirement. In an embodiment that does not employ a loop count,
step 214 can notify the operator that accelerometer testing is
being performed each time this step is entered. If the loop count
is not exceeded at 238, operation proceeds to 240 which increments
the loop count and sets the priority order counter to 1. Operation
then returns to 210 and proceeds therefrom. On the other hand, if
238 determines that the loop count exceeds a loop count limit,
which can be established, for example, by the manufacturer, a
warning can be issued to the operator at 244.
[0043] The method and associated apparatus described above can
readily be used with additional accelerometer packages having any
suitable number of sensing axes and/or a single sensing axis for
even more redundancy. Further, the procedures of FIGS. 4 and 7 are
not limited to tri-axial accelerometer packages and the
accelerometer packages are not required to be normally mounted with
respect to one another so long as three Cartesian acceleration
directions can be resolved from the accelerometers selected, as
will be described in detail immediately hereinafter.
[0044] Attention is now directed to FIG. 8 which is a diagrammatic
illustration, in a perspective view, of another embodiment for
supporting dual accelerometers in accordance with the present
disclosure. It is noted that the accelerometer axes are shown
independent of physical packaging and the axes are indicated using
reference designations that are taken from FIGS. 1 and 2. First
circuit board 50 and second circuit board 52 are shown as planes
for purposes of diagrammatic simplicity while accelerometers 44a
and 44b are considered as being located at the origins of their
respective coordinate axes. In the present embodiment, second
circuit board 52 is supported at an acute angle .beta. relative to
first circuit board 52. The angle .beta. can have any suitable
value. In one embodiment, .beta. can be at least approximately
45.degree.. The x.sub.2 axis can be at least approximately aligned
or parallel with the elongation axis of the transmitter, as seen in
FIG. 1, although this is not a requirement. The in-plane axes,
x.sub.1 and y.sub.1 of accelerometer 44a in FIG. 1 remain at least
approximately parallel to first circuit board 50 but have been
rotated by an angle. The z.sub.1 axis is at least approximately
orthogonal to the plane of circuit board 50. In the present
embodiment, angle .alpha. is at least approximately equal to
45.degree.. In other embodiments, any suitable angle can be
utilized for .alpha.. For example, .alpha. can be in the range from
20.degree. to 160.degree. which will allow for a sufficient
projection onto axes of interest.
[0045] Still referring to FIG. 8, it should be appreciated that
angles .alpha. and .beta. can be determined at the time of
manufacture and/or determined based on a calibration procedure that
is performed, for example, when the transmitter and accelerometer
arrangement are supported in a known physical orientation. The
calibration procedure can orient the transmitter in six cardinal
orientations based on three orthogonal axes that can be referenced
to the housing of the transmitter. These cardinal orientations can
correspond to roll positions of 0.degree., 90.degree., 180.degree.
and 270.degree. with a pitch angle of 0.degree., and pitch angles
of +/-90.degree.. In this way, angles .alpha. and .beta. can be
determined as well as any angular variation of axis x.sub.2 from
the elongation axis of the transmitter. The orientation of each
axis can be characterized in a well-known manner, for example,
based on Euler angles that utilize the transmitter elongation axis
and a zero roll orientation, as references. Based on the described
values for angles .alpha. and .beta., each axis of accelerometer
44a is skewed or exhibits an angular offset with respect to every
axis of accelerometer 44b. As will be seen, the accelerometer
arrangement depicted in FIG. 8, and variations thereof, provide a
significant number of combinations of axes and flexibility for
purposes of measuring roll and pitch orientation.
[0046] Table 3 sets forth the combinations of axes that can be used
for roll and pitch orientation in accordance with the embodiment of
FIG. 8. It is noted that for each combination, two axes are used
for detecting the roll orientation and a different axis is used for
detecting the pitch orientation. For a particular combination, the
axes used for detection of roll orientation are designated using an
"R" and the axis that is used for detection of pitch orientation is
designated as "P". As listed in the final column of the table, the
pitch measurement can be sensitive to angle .beta. or angles
.alpha. and .beta. when the axis that is used for pitch measurement
is skewed with respect to axis x.sub.2, which is assumed for
purposes of the present example to be parallel to the elongation
axis of the transmitter. Since axis x.sub.2 is assumed to be
parallel to the elongation axis, no such sensitivity is exhibited
(designated as "N/A" in the table) with respect to pitch
measurement using x.sub.2.
TABLE-US-00003 TABLE 3 ACCELEROMETER AXIS COMBINATIONS FOR PITCH
AND ROLL Roll Pitch sensitivity proportional Combination x.sub.1
y.sub.1 z.sub.1 x.sub.2 y.sub.2 z.sub.2 to: 1a P R R N/A 1b P R R
sin .beta. 1c P R R cos .beta., sin .alpha. 1d P R R cos .beta.,
cos .alpha. 2a R P R N/A 2b R P R cos .beta., cos .alpha. 2c R P R
sin .beta. 3a R P R N/A 3b R P R sin .beta. 4a R P R N/A 4b R P R
cos .beta., cos .alpha. 4c R P R sin .beta. 5a R P R N/A 5b R P R
sin .beta. 5c P R R cos .beta., sin .alpha.
[0047] Accordingly, fifteen different combinations are available.
It should be appreciated that these combinations can be
prioritized. For example, combinations that rely on either z.sub.1
or z.sub.2 can be assigned a relatively lower priority than
combinations that do not rely on these axes. Combinations that rely
on z.sub.1 and z.sub.2 can be assigned still lower priority.
Applicants submit that the wide range of combinations of axes in
Table 1 can provide for significant immunity with respect to the
failure of one or more accelerometer axes in terms of pitch and
roll measurement. It is noted that 14 out of the combinations given
in Table 3 utilize outputs from both triaxial accelerometers.
[0048] Still referring to FIG. 8, additional combinations can be
used which utilize all three axes of each accelerometer. In these
embodiments, the orthogonal x, y and z accelerometer measurements
taken by either accelerometer 44a or 44b can be resolved onto
reference axes of the transmitter, for example, onto the
transmitter elongation axis and an axis that corresponds to a zero
roll orientation. In the example of FIG. 8, the transmitter
elongation axis is additionally designated as Gx and corresponds to
axis x.sub.2 under the assumption that x.sub.2 is at least
approximately parallel thereto. A roll orientation reference axis
Gy corresponds to axis y.sub.2, assuming that the transmitter is
oriented at a roll reference position of zero degrees, and a
reference axis Gz that is orthogonal to both Gx and Gy. With these
values in hand and in one embodiment, the roll orientation can be
determined based on:
Roll=atan 2(Gy,Gz) EQN (5)
[0049] It should be appreciated that the function atan 2 is an
arctangent function with two arguments which returns the
appropriate quadrant for the roll angle that is determined.
[0050] In another embodiment, the roll orientation can be
determined based on
Roll = arc tan [ Gy Gx 2 + Gz 2 ] EQN ( 6 ) ##EQU00003##
[0051] Accordingly, even more flexibility is provided based on
Equations 5 and 6 with respect to the ability to determine roll
orientation.
[0052] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form or forms disclosed, and other modifications and variations may
be possible in light of the above teachings. Accordingly, those of
skill in the art will recognize certain modifications,
permutations, additions and sub-combinations of the embodiments
described above.
* * * * *